1. Field of the Invention
This invention concerns polymeric films and more particularly polypropylene films having good opacity and gloss.
2. Description of the Prior Art
Polypropylene films can be made opaque by including an opacifying pigment and/or by including a particulate. material in the polymer used to form the films and which gives rise to opacifying voids within the films when they are stretched. Typically chalk having a mean particle size of about 3 .mu.m is used to induce opacifying voids, stretching usually being effected biaxially and sequentially in the longitudinal and transverse directions.
Voided polypropylene films are usually provided with one more outer heat sealable layers. In addition to imparting heat sealability, heat seal layers do tend to improve the surface gloss of voided films, but their gloss is often poor compared with that of analogous non-voided films.
The gloss of voided films can, however, be improved by the inclusion of a layer of non-voided polypropylene homopolymer between the voided layer, which is usually the core layer of the film, and the outer heat seal layer. The disadvantage of doing so is, though, that-the opacity of the resultant films is relatively poor because the voided layer represents a smaller proportion of the total film thickness, and the yield of film, i.e. the area of film of a given thickness derived from a given weight of polymer, is reduced. The addition of a pigment, for example titanium,dioxide, to the voided layer or to the non-voided layer can increase the opacity of the films, but it does not increase film yield.
More recently films have been proposed which are said to have good gloss combined with high opacity by the use of chalk having a particle size of 1-2 .mu.m as the voiding agent, the films having outer heat seal layers and intermediate layers of polypropylene homopolymer. However, the yield of film is relatively low compared with films which don't include the intermediate layers, but omitting the intermediate layers leads to reduced gloss.